In non-destructive testing (NDT) of large structures, in particular wind turbine blades, a non-destructive testing readout corresponds to a non-destructive testing probe position. Often the exact position is required for further applications, such as repairs or inspections. Possibly such applications are realised at a later date than the non-destructive testing.
Generally the determination of the position of a non-destructive testing probe and the recovery of a particular position can be realised by optical or other marking on the structure where the original testing has been performed. Such marking should unambiguously correlate to a particular non-destructive testing readout and should be recognisable for different persons. Moreover, the marking should be non-destructive, but also resistant against different environmental influences. These requirements are difficult to reach.
Another possibility is the use of large two-dimensional scanners, for instance arrays of non-destructive testing probes that are guided automatically over the surface to be scanned. This arrangement requires the positioning of the structure to be tested in a specific fixture, which reduces flexibility and causes risk of constraints.
A modern wind turbine typically comprises a tower, a nacelle which is rotatably mounted onto the tower, and a rotor which is mounted onto the nacelle. Criteria for concept development and improvement of wind turbines are a high efficiency, minimal acoustic emission, small material usage, and a high lifetime. Large wind turbines are installed off-shore because of the extensive foundation. Such wind turbines typically have a rotor with a diameter up to 130 m. Especially the rotor blades are exposed to enormous load changes. The rotor blades typically have a lifetime between 15 and 20 years. The increasing rotor diameters require improved material properties.
Rotor blades are usually made of composite materials, for instance glass-fibre reinforced plastic (GRP). During the manufacturing process it is difficult to achieve a force-fit connexion between all inner components, for example between the upper and lower housing. It can be necessary that the rotor blade body has to be evacuated and subsequently be immersed in resin. Thereby air inclusions may occur, which may decrease the stability of the rotor blade. In this case delaminations can occur or cracks may be formed, especially at high loads.
The quality of rotor blades is usually controlled after finishing the manufacturing process and after mounting the rotor blades onto the hub of the wind turbine. After mounting the control is performed by optical means, for example by making photos of the observed defects. In this case it is not possible directly to compare the material properties of the rotor blades after manufacturing and after mounting onto the wind turbine.
In DE 102 59 653 B3 a method and a device for non-destructively testing a work piece by means of ultrasound is disclosed, wherein the work piece is scanned by sound using at least two synchronised transponders for ultrasound which are arranged opposite to each other at different sides of the work piece. The transponders for ultrasound and/or the work piece can be moved in relation to each other.
In PEPPERL+FUCHS GmbH “VDM35-30R/20/105/122 Montage-und Bedienungsanleitung” 14 Mar. 2006 (Mar. 14, 2006), PEPPERL+FUCHS GmbH Mannheim, XP002445384, a laser positioning system with a reflector which works in the necessary distance range is disclosed. The laser emits directional radiation, and the direction can be controlled by simply turning the device.
In DE 10 2004 044 342 B4 a method and a device for automatically testing a beam welded connexion between a duct and a stub by means of ultrasound are disclosed. The used ultrasonic testing probe can be moved along the work piece, especially along the weld, according to previously recorded data concerning the geometry of the work piece.
In DE 103 32 457 A1 a method and a device for determining the position of a moving object by measuring the propagation delay of waves are disclosed. The waves are emitted from a transmitter assigned to the moving object. The emitted waves are received by at least two receivers at a known position.
In WO 2005/119054 A1 a method and a device for performing quality control of a rotor blade of an electricity-producing wind power station is disclosed. The device is configured so as to travel along and examine the rotor blade in an automatic or remote-controlled manner.
In DE 10 2004 061 870 B3 a sensor wheel for the acoustic inspection of a measuring object is disclosed. Said sensor wheel comprises an ultrasound-permeable and sufficiently solid hollow roll that is provided with two rim rings and has a bearing surface that can be placed on the measuring object. The sensor wheel is provided with two wheel discs and an axle that has a cylindrical container that is radially flared outward inside the sensor wheel and that has an at least partially ultrasound-permeable surface of the cylinder. The sensor wheel also comprises an ultrasound-permeable liquid coupling agent between the roll and the container, said container having at least one ultrasound-producing sound transducer. The ultrasonic waves can be emitted through the coupling agent, the roll and the bearing surface into the measuring object. The sensor wheel is suitable for use in the inspection of railroad tracks and/or railroad wheels.